Hey there!
You can find the rate of change by taking any two points from your graph and plugging them into the following equation:
Let's use the first two points you're shown from least to greatest. These are (0, 0) and (1, 15).
Now, identify what number is which and plug them into your equation. The solution will be your rate of change.
15, or D, will be your answer.
Hope this helped you out! :-)